A. Field of the Invention
The invention relates to energy receiving apparatus, particularly of the collecting type whereby radiation is absorbed by or emitted from the collecting apparatus. The invention is particularly useful for efficiently collecting solar energy at temperatures sufficiently high to permit effective conversion of the collected solar energy to other useful forms of energy.
B. Description of the Prior Art
Solar receivers and collectors of the absorbing type are generally limited in their performance at high temperatures by reradiation losses which are directly proportional to the fourth power of the temperature of the apparatus. Prior art solar energy collecting apparatus have included "cavity absorbers", such as are disclosed in U.S. Pat. Nos. 3,208,447; 2,793,018; and 2,760,920; which absorbers are comprised of "silvered" tubular units having an "entrance" end located at the focus of an optical system for concentrating the sun's light into an absorbing "black body" cavity internal of the tubular unit. However, reradiation loss from a cavity absorber of this type at temperatures sufficiently high to be useful in Carnot engines, turbines, or the like is comparable to the energy entering the absorber due to the T.sup.4 effect mentioned above. Thus, cavity absorbers have proven to be particularly inefficient at the relatively high temperatures required for efficient Carnot cycle operation. U.S. Pat. Nos. 3,217,702 and 2,872,915 provide means for reducing reradiation loss by reflecting at least a portion of this loss back into the cavity. However, the efficiency of cavity absorbers has not been appreciably increased until the conception of the present invention wherein reradiation from a cavity absorber is substantially prevented rather than merely recovered in part. In effect, the present invention provides inexpensive apparatus useful with economical solar concentrating apparatus for efficiently converting incident solar energy to heat energy at a temperature sufficiently high to perform useful work.